The present invention relates to an apparatus for protecting a sensor. In particular, the invention provides an apparatus for protecting a magnetic resonance antenna used for measuring the parameters of an earth formation while drilling a borehole into the formation.
The measurement of desired earth parameters during the actual drilling process is known in the art as measuring while drilling or logging while drilling. Sensors located inside or outside of a drill collar perform measurements on the rock and its fluid content. The measurements are either stored downhole for later retrieval or transmitted to the surface in real-time. The drill collar is a metallic structure that conveys the torque required for the drilling operation. Also, the drill collar is a conduit for the drilling mud that is used to lubricate the drill bit and carry the cuttings to the surface. Since audio and radio frequency electromagnetic fields do not penetrate the metallic body of the drill collar, sensors of electromagnetic fields must be mounted outside the metallic body of the drill collar. These sensors are subject to abrasions resulting from particles in the drilling mud and the impact of the sensor against the earth formation. Often, wear bands are employed on the drill collar to provide an adequate amount of standoff between the sensors and the formation thereby reducing or eliminating the impact of the sensor against the earth formation.
U. S. patent application Ser. No. 08/430,697 describes a slotted metallic shield covering a logging while drilling nuclear magnetic resonance tool. However, a slotted metallic shield significantly degrades the sensitivity of a radio frequency electromagnetic sensor. The loss of sensitivity is detrimental in magnetic resonance measurements, which require the detection of weak signals.